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The effect in which a chelate complex is generally more stable than the analogous complex formed with monodentate ligands. For example, the complex ion [Cu(en) (OH2)4]2+ is more stable than the complex ion [Cu(NH3)2 (OH2)4]2+. Here, en denotes the bidentate ethylene diamine (1,2-diaminoethane) ligand. The main cause of the chelate effect is the effect of reaction entropy when the complex is formed. Thus, the reaction [Cu(OH2)6]2++en → [Cu(en)(OH2)4+2H2O results in a net increase in the number of molecules (from 2 to 3). The reaction [Cu(OH2)6]2++2NH3 → Cu(NH3)2(OH2)4+2H2O involves no net increase in the number of molecules. As a result, the chelate reaction has a larger reaction entropy and is more favourable.